The process of transformation of hides into leather is one of the oldest human activities, existing since prehistoric times, when the hides were used as clothing for protection against the cold.
The first tannage, which is the process for transforming the hide, a putrescible material, in leather, a material that is stable and resistant to microorganisms, probably happened by chance. Some hides may have been placed in contact with tannin-rich plants, and the tanning may have been caused thereby through the action of rainfall.
In the present days, the hide tanning industry uses advanced technologies, thereby allowing to achieve a high level of productivity. To this end, among other aspects, there contribute the so called tanning agents, which are substances of plant, mineral or synthetic origin that will interact with the collagen fibers of the hide, thereby promoting the tanning thereof.
There should be pointed out the mineral agents, such as chrome salts, and particularly chrome sulfate. The synthetic agents are basically comprised by naphthalene sulphonic acids or salts and compounds derived from carbolic acid. In the category of tanning agents obtained from plants, a great variety of plants evidence the presence of substances with tanning properties.
The presence of tanning substances occurs in practically every plant in the vegetable kingdom, and especially in the superior plants. The localization of the tannin in the plants is equally varied, as it may be present in the trunk, the leaves, the bark and the roots. The occurrence thereof takes place in isolated cells or in cell groups, or yet in special cavities of the plant, and it commonly occurs in the cytoplasm of the plant cells.
The various plant extracts provide to the final products of tannage of hides, the tanned leathers, various characteristics according to the plant wherefrom the plant extracts were obtained.
However, the amount of plant species which are economically feasible to exploit is very restricted, and depending on the species the amounts of tannins present therein, as well as the characteristics of such tannins and the conditions related to the plant exploitation itself, may exhibit variation.
Among the main species there are included the Acacia Mimosa (Acacia mearnsii de Wild), the Quebracho (Schinopsi lorentzi), the Chestnut (Castanea Vesca), zumagre, gambir among others. Of these, only the Acacia Mimosa has a production system exclusively based on plants obtained by reforesting methods.
Chemically, the tannins are classified in two groups, according to their chemical structure: (i) the condensed tannins and (ii) the hydrolyzable tannins.
The hydrolyzable or pyrogallic tannins are polyester structures easily hydrolyzable by the action of strong acids or yet of enzymes. As a result of this hydrolysis there are obtained sugars, alcohols and phenol carboxylic acid. The phenol carboxylic acid will dissociate into gallic acid and ellagic acid. The extracts of Chestnut constitute typical examples of hydrolyzable or pyrogallic tannins.
The tannins classified as condensed tannins are also called catecholic tannins or phlobatannins. They are comprised of flavonoid units of the type flavan 3,4-diol and flavan 3-ol, with various degrees of polymerization therebetween. When treated with strong acids under heat, they start a progressive polymerization process, up to complete polymerization, originating amorphous tannins, known as phlobaphenes. The extracts of Acacia Mimosa are characteristic examples of catecholic type tannins or phlobatannins.
The extracts of Acacia Mimosa, which are of the catecholic tannin or phlobatannin types, are comprised of flavonoid units of the type flavan 3,4-diol and flavan 3-ol, with various degrees of cross-polymerization. The chemical binding between the tannin of this group and the collagen fibers occurs by interactions of the hydrogen binding type, between the phenolic hydroxylic groups and certain groups associated to the polypeptidic chain. Those extracts of the catecholic tannin or phlobatannin types evidence fast penetration into the hide, good dispersion among the collagen fibers, high resistance to electrolytes, a tanning rate of about 70 to 74%, a rate of non-tanning fraction of about 20 to 25%, an astringency of about 3.5 (ratio of tanning agents to non-tanning matter).and a pH value of about 4.8 to 5.2.
The extracts of Quebracho are of the condensed type and the maturation of this species is completed at the age of 80 years, whereupon the yield in tannin is particularly advantageous. Those extracts exhibit a high content of non-soluble matter and a high astringency (ratio of tanning agents to non-tanning matter).
The process of chemical modification of the extracts of Acacia Mimosa has carried out for many years, using sulphites and bisulphites usually in the form of sodium salts, for the purpose of altering the solubility of the extracts and particularly the color of these extracts.
Regarding past developments, the present inventors may cite as prior art examples the following papers: Belavsky, E.—O Curtume no Brasil (Tannage in Brazil)—Porto Alegre, Editora Globo, 1965; Bienkiewica, K—Physical Chemistry of Leather Making—Robert E. Krieger Publishing Co., Inc.—USA, 1993; Hoinacki, E.—Peles e Couros (Skins and Hides)—2a Edição (2nd Edition) SENAI/RGS—Porto Alegre, 1989; Howes, F. N.—Vegetable Tanning Materials, London, Butterworths Scientific Publications, 1953; Leather Industries Research Institute—Wattle Tannin and Mimosa Extract—Grahamstown, South Africa, 1955; Mugica, M. G. & Ochoa, J. T. Los Taninos Vegetales (Plant-Derived Tannins), Madrid, Instituto Florestal de Investigaciones Y Experiencias, 1969; O'Flaherty, F.; Reddy, W. T.; Lollar R. M.—The Chemistry and Technology of Leather—Vol. IV—Evaluation of Leather—Reinhold Publishing Corporation, New York, 1965; Pizzi, A.—Wood Adhesives—Chemistry and Technology—Marcel Becker, Inc.—New York, 1983; Sherry, S. P.—The Black Wattle—University of Natal Press—South Africa, 1971; TEPF—Tanning Extract Producers Federation—A Survey of Modern Vegetable Tannage—England, 1974.
According to Pizzi, in Wood Adhesives—Chemistry and Technology—1983, the sulphitation of the tannin present in the extracts is one of the oldest and most widely used reactions in flavonoid chemistry. As a rule, the sulphitation provides to the tannins a decrease in viscosity and an increase in solubility. Both these effects are due to the following factors:
1. The elimination of the heterocyclic ether group, which is water-repellent, and as known by the specialists the aqueous extracts of tannin are not actual solutions but rather hydrocolloidal suspensions, wherein a part of the tannin molecule maintains the tannin solubilized while the other part tends to separate the tannin from the solution.
2. The introduction of the sulphonic group and one other hydroxyl group, both hydrophilic.
3. Decrease in the rigidity of the polymer, an esteric restriction, and the intermolecular hydrogen bonding resulting from the opening of the heterocyclic ring.
4. The acid hydrolysis of the hydrocolloidal starches and the interflavonoid bonds.
The following is a typical reaction in the process according to the present invention: 